各种google没找到完整的判断星期几的demo，参考部分前辈的代码，自己琢磨写了个方法，大神看了莫拍。
- (NSString*)weekdayStringFromDate:(NSString*)inputDate {
//截取年月日
NSString *year = [inputDate substringToIndex:4];
int yearNum = [year intValue];
NSString *mon0 = [inputDate substringFromIndex:4];
NSString *mon = [mon0 substringToIndex:2];
int monNum = [mon intValue];
NSString *day = [inputDate substringFromIndex:6];
int dayNum = [day intValue];
//定义一个NSDateComponents对象，设置为导入的时间点
NSDateComponents *_comps = [[NSDateComponents alloc] init];
[_comps setDay:dayNum];
[_comps setMonth:monNum];
[_comps setYear:yearNum];
//一周的数组
NSArray *weekdays = [NSArray arrayWithObjects: [NSNull null],@"星期天", @"星期一", @"星期二", @"星期三", @"星期四", @"星期五", @"星期六", nil];
//定义一个遵循某个历法的日历对象
NSCalendar *gregorian = [[NSCalendar alloc]
initWithCalendarIdentifier:NSCalendarIdentifierGregorian];
NSDate *_date = [gregorian dateFromComponents:_comps];
NSDateComponents *weekdayComponents =
[gregorian components:NSCalendarUnitWeekday fromDate:_date];
int _weekday = [weekdayComponents weekday];
//返回星期几
return [weekdays objectAtIndex:_weekday];
}